The present invention relates to a device in hammer machines for drilling, preferably in rock, by means of a drill string comprising a set of tubes and a set of rods arranged in the set of tubes.
When drilling with drill strings of the above mentioned kind, see SE-A-8900590-4, one has used drill strings where the drill bit and the rod closest to the drill bit together have a length which is equal to the length of the hammer piston in order to efficiently transfer the energy of the hammer piston to the drill bit. The rod adjacent to the drill bit is provided with a channel which connects the external surface of the rod with a centrally positioned channel through which flushing medium is supplied to the drill bit. It has turned out that this build-up of the drill string in drilling creates large stresses in the rod at the channel between the external surface and the central channel. One reason for this is that the shock wave from the hammer piston has not passed the channel between the external surface and the central channel before the reflex from the rock reaches this section.